Waveguides can generally be classified as "fundamental mode" or "overmoded"; see, for example, the test by A. E. Karbowiak entitled Trunk Waveguide Communication, published by Chapman and Hall Ltd. (1965). A fundamental mode waveguide is designed with dimensions which support only the fundamental electromagnetic field, or mode, configuration for propagation in a given frequency band, i.e., higher-order modes are in a "cutoff" condition. An overmoded waveguide, however, is designed so that several or many modes could be supported, but internal structures are generally provided to suppress all but the desired modal configuration. Fundamental mode waveguides (hereinafter referred to as "conventional waveguide") are far more common, as it is more easily designed and constructed; however, this waveguide is restricted in maximum power capacity and in minimum loss, because of its required cross sectional dimensions. Overmoded waveguide, on the other hand, can be designed to have arbitrarily high power capacity and arbitrarily low attenuation by appropriately increasing the cross section. As described in the aforementioned text by Karbowiak, required suppression of unwanted modes in overmoded waveguides is achieved using dielectric and metallic structures to restrict allowable modes.
Overmoded waveguide has been applied as telecommunications trunk transmission lines and to connect transmitters to communications or radar antennas; see W. D. Warters article entitled "WT4 Millimeter Waveguide Systems: Introduction" Bell Systems Technical Journal, Vol. 56, No. 10, December 1977, pp. 1825-1827 and that of R. M. Collins entitled "Practical Aspects of High Power Circular Waveguide Systems", NEREM Record 1962, pp. 182-3. As noted previously by Karbowiak, an important type of overmoded waveguide supports the circular TE.sub.01 mode which has the unique property of decreasing transmission loss with increasing frequency for a given diameter. Although applied most often to exploit this low-loss characteristic, the potential for overmoded waveguide to support much higher power than conventional waveguide has also been considered; see, for example, the above-noted NEREM Record article by R. M. Collins, as well as the article by W. Lowenstern, Jr. and D. A. Dunn entitled "On the Feasibility of Power Transmission Using Microwave Energy in Circular Waveguide", appearing in the Journal of Microwave Power, Symposium Proceedings, Part B., Vol. 1, No. 2 (1966) pp. 57-61.
A disadvantage of overmoded waveguide is that the associated bends and elbows are larger than their conventional counterparts, primarily to minimize mode conversion; thus, posing a problem for systems with space limitations. As will be described hereinafter, a more compact mode-transducing bend is proposed in accordance with the present invention, employing a high power capacity mode transducer which efficiently couples an overmoded circular waveguide to four or more conventional rectangular waveguides; i.e. a TE.sub.01 mode overmoded waveguide is transitioned into multiple, smaller cross section, conventional waveguides which are sharply bent and then re-transitioned to an overmoded waveguide. As will also be described in more detail hereinafter, a conventional waveguide bend can be more compact because the cross section is smaller and the mode conversion loss mechanism is not present. Thus, the mode-transducing bend, or elbow, proposed in accordance with the present invention takes advantage of the desirable features of overmoded waveguide, i.e., high power and relatively low loss, while also featuring a desirable characteristic of conventional waveguide, i.e., compactness.
Currently available TE.sub.01 mode overmoded waveguide elbows and bends can be classified into three basic types: TE.sub.01 mode gradual bends, miter elbows, and mode-transitioning gradual bends. The most common, and probably highest-performance, type is the overmoded TE.sub.01 mode bend design which has been extensively analyzed and optimized; see for example, T. N. Anderson article entitled "State of the Waveguide Art", Microwave Journal, Vol. 25, No. 12, pp. 22-48 (December 1982), as well as the Bell System Technical Journal, Vol. 28, No. 1, pp. 1-33 (January 1947); Vol. 36, No. 5, pp. 1292-1307 (September 1957); and Vol. 37, No. 6, pp. 1599-1663 (November 1958). In particular, the unwanted-mode suppression features of this particular bend configuration include a gradual curvature taper and a wall structure, generally in the form of corrugations, insulated helix wire sheathed in dielectric, or a dielectric lining. Such a bend can be 10-100 times longer than a conventional waveguide bend, depending on the required loss and mode conversion characteristics (which influence cross section and curvature taper). Whereas the larger size may be of little concern at millimeter wavelengths or in the context of increased performance, it may preclude application of overmoded waveguide in many cases, especially at longer wavelengths.
A TE.sub.01 mode miter elbow has also been previously developed. In its simplest form, two orthogonal circular waveguides are joined with a mirror replacing the outer corner of the intersection, and energy from one waveguide is reflected by the mirror into the other waveguide, see E. A. J. Marcatili article entitled "Miter Elbow for Circular Elective Mode", Symposium of Quasi-Optics, Polytechnic Inst. Of Brooklyn, (June 8-10, 1964), pp. 534-543. This particular elbow performs better, and is generally analyzed, in the optical regime, with the waveguide may free space wavelengths in diameter. For less overmoded systems (i.e., smaller diameter relative to a wavelength), a low-loss miter elbow requires flaring the waveguide to a larger diameter at the intersection, as described by F. Sporleder in his paper "A Compact 90.degree. Corner with Expanded Diameter and Elliptic Mirror for Circular Waveguide", IEE Conference Publ. 146, (1976), pp. 68-71. Therefore, an overmoded miter elbow is generally not compact as compared relative to a conventional waveguide bend.
As reported by D. A. Lanciani in an article entitled "H.sub.01 Mode Circular Components", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-2, pp. 45-55, (July 1954), a gradual mode-transducing bend has also been previously constructed which transitions from the circular TE.sub.01 mode to the cross-shaped TE.sub.22.sup.+ mode, which is the intermediate mode in the well-known Marie transition (see U.S. Pat. No. 2,859,412). The portion of the bend with the cross TE.sub.22.sup.+ mode has a gradual curvature which preserves the fields in each arm of the cross, to allow proper transitioning back to the TE.sub.01 mode. The curvature requirements appear similar to those for the TE.sub.01 mode bend, and therefore, the bend radius is significantly larger than that of conventional waveguide bends.
In summary, the above discussed prior art types of bends for overmoded TE.sub.01 waveguide are all, in principle, capable of high power and low loss; however, they require large effective bend radius (or volume in the case of the miter elbow) as compared to conventional waveguide.